The WSJ article quoted unnamed US defense officials, who said that similar attacks had also penetrated the Air Force's air-traffic-control system, in a series of attacks over recent months:

In the case of the fighter-jet program, the intruders were able to copy and siphon off several terabytes of data related to design and electronics systems, officials say, potentially making it easier to defend against the craft... In addition, while the spies were able to download sizable amounts of data related to the jet-fighter, they weren't able to access the most sensitive material, which is stored on computers not connected to the Internet.

The identity of the attackers and the scale of the damage is unknown. But if accurate, the report could be an indication that the US military is vulnerable to cyber-attacks, launched either by independent operators or countries such as China, which has ramped up its so-called "online warfare" techniques. Particularly distressing is the hackers' alleged target: the F-35 Lightning II, a next-generation, radar-evading fighter, currently being manufactured by Lockheed Martin, Northrup Grumman, and BAE Systems. Although the Obama administration has sought to pare back on the purchasing of fighter jets, the F-35, which is slated to be formally unveiled in 2011, has received a good deal of attention in recent years, not least because of its price tag. The F-35 will be the costliest fighter ever produced by the Pentagon.

According to the official Joint Strike Fighter website, the plane will utilize a Electro-Optical Targeting system, an integrated avionics suite, and "the most powerful fighter/attack turbofans in the world." Watch the Lockheed Martin video here: